1. Field of the Invention
The present invention relates to a demodulator for demodulating an angle modulated signal such as a frequency modulated signal and, more particularly, to a demodulator suited for use in a receiver operated by a low voltage power source.
2. Description of the Prior Art
Angle modulation is divided into frequency modulation and phase modulation. The present invention will be described in the following in the case of the frequency modulation.
As the detecting system for a frequency modulated signal (hereinafter referred to as an FM signal), there has been widely known a Foster-Seely discriminator or a ratio detector, which is, however, not suited for a semiconductor integrated circuit. Recently, a desire has arisen for an FM detecting system which can be formed in a semiconductor integrated circuit. In order to meet this desire, a quadrature detecting system has been developed and is disclosed in U.S. Pat. No. 3,667,060 granted to Jack Avins. According to that disclosure, the FM signal is applied to a first differential amplifier and is phase-shifted and applied to a second differential amplifier, which is to be supplied with its emitter current from one of the outputs of the first differential amplifier, so that a demodulated signal is generated from the output of the second differential amplifier.
Since the quadrature detecting system thus far described requires a number of transistors connected in cascade between the power supply terminals, a defect of the system is that the power supply voltage necessary for normal operation is high. More specifically, the quadrature detecting system can exhibit excellent performance, such as a distortion-free operation, when used in a home stereo system which can have a sufficient power supply voltage, such as 12 V. However, the performance of the quadrature detecting system deteriorates when it is operated at a power supply voltage as low as 2 to 6 V, i.e., by which a portable radio receiver or a portable radio receiver with a cassette tape recorder is operated.